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Abstract
Paralia sulcata (Ehrenberg) Cleve is a brackish to marine diatom commonly found in coastal waters. In Saanich Inlet, Vancouver Island, British Columbia, P. sulcata is a dominant species in both the phytoplankton and sediments. The stratigraphy of this species from sediment cores shows an increase in absolute abundance occurring at approximately 6,000 cal 14C y BP. This change is likely to be associated with increases in sea level which would have flooded several bays in and around the inlet. These bays greatly expanded the available habitat for tychoplanktonic diatoms such as P. sulcata and are still an important seed source of this species to the plankton. Thus, the dominant factor controlling the increased abundance of P. sulcata appears to be changes in the shoreline rather than other environmental factors. Valve diameters of P. sulcata did not appear to change among different seasons; however, a significant decrease in cell diameter was observed for cells in the mid- to late Holocene corresponding with climatic cooling to near present temperatures. The data also could suggest a change in salinity. Our results emphasize the importance of considering the tychoplanktonic nature of P. sulcata when interpreting its usefulness as a palaeoindicator of environmental change.
